Method of making an integrated electromechanical switch and tunable capacitor

ABSTRACT

A monolithically integrated, electromechanical microwave switch, capable of handling signals from DC to millimeter-wave frequencies, and an integrated electromechanical tunable capacitor are described. Both electromechanical devices include movable beams actuated either by thermo-mechanical or by electrostatic forces. The devices are fabricated directly on finished silicon-based integrated circuit wafers, such as CMOS, BiCMOS or bipolar wafers. The movable beams are formed by selectively removing the supporting silicon underneath the thin films available in a silicon-based integrated circuit technology, which incorporates at least one polysilicon layer and two metallization layers. A cavity and a thick, low-loss metallization are used to form an electrode above the movable beam. A thick mechanical support layer is formed on regions where the cavity is located, or substrate is bulk-micro-machined, i.e., etched.

This application is a divisional of application Ser. No. 10/147,300,filed May 17, 2002 now U.S. Pat. No. 6,800,912, which claims the benefitof Provisional Application No. 60/291,423, filed May 18, 2001, theentire contents of which are hereby incorporated by reference in thisapplication.

FIELD OF THE INVENTION

The present invention relates generally to tunable and re-configurablemicrowave systems, and, in particular, to the fabrication ofre-configurable silicon-based integrated circuits, with integratedelectromechanical switches and capacitors.

BACKGROUND OF THE INVENTION

Microelectromechanical switches (MEMS) have been shown to have very lowlosses at very high frequencies. Compared to traditional activemicrowave switches based on transistors or diodes, the quality factors(i.e., 1/R_(on) C_(off), where R_(on) is the resistance of the switch inthe ON-state and C_(off) is the capacitance in the OFF-state) of MEMSswitches are very high. Therefore, MEMS microwave components aresuitable for in many types of applications.

High quality MEMS switches enable the construction of electrical systemswith greatly improved functionality and flexibility. Such systems can beelectrically re-configured to perform many different electricalfunctions without a loss of significant operating quality. However, ifthe electromechanical switches, control circuitry for the switches, andconductive traces among which the electrical reconfiguration is done arefabricated on different substrates, such benefits would not be assignificant. Monolithic fabrication is very important for achieving thequality, reliability, functionality, and low-cost of such MEMS systems.

SUMMARY OF THE INVENTION

The present invention is directed to a low-loss micro-electromechanicalmicrowave switch and a micro-electromechanical tunable capacitormonolithically integrated with low-cost silicon-based integratedcircuits. The microwave switch of the present invention is capable ofhandling signals from DC to millimeter-wave frequencies. Both the switchand tunable capacitor include movable beams actuated either bythermo-mechanical or electrostatic forces. The movable beams are formedby selectively removing the supporting silicon underneath the thin filmsavailable in a silicon-based integrated circuit technology, whichincorporates at least one polysilicon layer and two metallizationlayers. A cavity and a thick, low-loss metallization layer are used toform an electrode above the movable beam. A thick mechanical supportlayer is formed in regions where the cavity is located, or the substrateis bulk-micromachined (i.e., etched).

The devices are fabricated directly on finished silicon-based integratedcircuit wafers, such as CMOS, BiCMOS or bipolar wafers. The presentinvention uses monolithic integration wherein the MEMS devices areconnected to the integrated circuits necessary to control theiroperation, the integrated circuits being on the same substrate as theMEMS devices they control. In the present invention, this processing isperformed on non-active circuit areas, i.e., where “passive” components,such as resistors, capacitors, inductors, interconnections, etc. arelocated. The functions and operation characteristics of active circuitsdo not change as a results of the process sequence of the presentinvention.

Reconfiguration capability is an advantage of the MEMS-IC integration ofthe present invention. For example, a frequency selective filter basedon MEMS devices, such as MEMS switches and MEMS tunable capacitorsand/or inductors allows the switches to switch-in (or out) selectedpassive component(s) to the circuit configuration of the filter. Byswitching in and out electrical components into the circuitconfiguration, the overall circuit can be changed. Thus, for example, apassive LC filter can be changed from low-pass filter to band-passfilter by switching-in a selected set of inductors and capacitors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows two perspective views of two halves of a preferredembodiment of the electromechanical switch of the present invention,where the switch has been split open to show its internal construction.

FIG. 2(a) is a perspective view of the bottom side of theelectromechanical switch shown in FIG. 1, where the substrate andsuperstrate mechanical support layers are not shown for ease inunderstanding the operation of the switch.

FIG. 2(b) is a perspective view of the top side of a preferredembodiment of the electromechanical tunable capacitor of the presentinvention, where the substrate and superstrate mechanical support layersare not shown for ease in understanding the operation of the switch.

FIG. 2(c) is a perspective view of the bottom side of theelectromechanical tunable capacitor shown in FIG. 2(b), where thesubstrate and superstrate mechanical support layers are not shown forease in understanding the operation of the switch.

FIG. 2(d) is a perspective view of another embodiment of theelectromechanical switch of the present invention using electrostaticactuation.

FIG. 3 is a graph showing deflection data for cantilever beams, such asthat used in the present invention.

FIG. 4 is a top plan view of the electromechanical switch shown in FIG.1.

FIG. 5 is a cross-sectional view of an electromechanical switch shown inFIGS. 1 and 4 taken along the section line 5—5 shown in FIG. 4.

FIG. 6(a) is a cross-sectional view of the electromechanical switch ofFIG. 4 taken along the section line 5—5 after a full, standard, singlepolysilicon, double metallization CMOS process sequence.

FIG. 6(b) is a cross-sectional view of the electromechanical switch ofFIG. 4 taken along the section line 5—5 after deposition and patterningof the sacrificial film which defines an air-cavity in which theswitch's beam moves.

FIG. 6(c) is a cross-sectional view of the electromechanical switch ofFIG. 4 taken along the section line 5—5 after deposition of the seedlayer necessary for electro-deposition of thick conductive films.

FIG. 6(d) is a cross-sectional view of the electromechanical switch ofFIG. 4 taken along the section line 5—5 after deposition and patterningof a mold necessary for electro-deposition of thick conductive films.

FIG. 6(e) is a cross-sectional view of the electromechanical switch ofFIG. 4 taken along the section line 5—5 after electro-deposition of athick conductive film.

FIG. 6(f) is a cross-sectional view of the electromechanical switch ofFIG. 4 taken along the section line 5—5 after deposition ofnon-conductive mechanical support layer before which the mold usedduring electro-deposition and the seed layer are removed.

FIG. 6(g) is a cross-sectional view of the electromechanical switch ofFIG. 4 taken along the section line 5—5, after deposition of a maskinglayer and patterning by front-to-back aligned lithography.

FIG. 6(h) is a cross-sectional view of the electromechanical switch ofFIG. 4 taken along the section line 5—5, after selective removal ofsilicon substrate through the mask.

FIG. 6(i) is a cross-sectional view of the electromechanical switch ofFIG. 4 taken along the section line 5—5, after removal of thesacrificial film which defines the air-cavity in which the switch's beammoves.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a perspective view of one embodiment of the electromechanicalswitch 10 of the present invention where switch 10 has been split opento show its internal construction. FIG. 4 is a top plan view of theelectromechanical switch 10 in FIG. 1. FIG. 5 is a cross-sectional viewof switch 10 taken along section line 5—5 shown in FIG. 4. Switch 10 isfabricated on a silicon wafer substrate 25, and includes a moveable beam12 that moves within a cavity 14 to contact a conductive metal bridge13. Deposited on top of substrate 25 is a superstrate 23 which supportsconductive bridge 13. FIG. 2(a) is a bottom view of a thermally-actuatedembodiment of switch 10, illustrated without the mechanical supportlayers, i.e., substrate 25 and superstrate 23, being shown for ease inunderstanding the operation of switch 10.

As shown in FIG. 2(a), switch 10 includes an n-shaped polysilicon heater20 and two traces 22 that are formed in a first metal layer (not shownas before etching). Traces 22 provide power to heater 20 throughconnections 21. Above traces 22 are metal traces 11 and 9 which aredeposited as part of a second level of metallization (also not shown asbefore etching). Traces 11 and 9 form microwave wave guides. Coplanarwaveguides are preferred because the ground planes 11 are formed in thesame plane as the signal plane 9. Deposited between these conductinglayers are dielectric layers 17, 18 and 19, which function as insulatinglayers. Layer 17 is a field oxide layer, while layer 18 is an insulatinglayer between the first polysilicon layer and the first metal layer.Layer 19 is an insulating layer between the first metal layer and thesecond metal layer. Layer 15 is an insulating layer that covers thesecond metal layer. FIG. 5 shows a cross-sectional view of device formedusing a one polysilicon layer and two metal layer CMOS process. Thenumber of interconnection layers, i.e., metal layers, can be increasedfor more complex designs, such as modern CMOS processes that producetens of millions of transistors in small areas which require as many asten metal interconnection layers.

Moving beam 12 of MEMS switch 10 is formed using a thin-film depositedduring IC fabrication. Moving beam 12 is a released layer, which, alongwith polysilicon heater 20, is fully released, except on one side.Deposited over second metal layer 11 and beam 12 is a dielectric layer15 which functions as an insulating layer. Directly above beam 12 is aconductive bridge 13 formed using a third layer metallization 33 (seeFIG. 6(g)), which is deposited as a part of the fabrication sequencedescribed in FIGS. 6(a) to 6(i). Conductive bridge 13 is electricallyconnected to ground plane 11 through a plurality of cuts 16 ininsulating layer 15. Bridge 13 is connected to ground plane 11 toachieve a shunt switching function, i.e., the signal line 40 isconnected and disconnected to ground plane 11 through bridge 13.

Beam 12 is mechanically free to move in a vertical direction. Because ofinternal mechanical stresses, beam 12 is typically curved away from thesurface of the silicon wafer 25 towards bridge 13. However, when beam 12is heated by applying voltage across the polysilicon heater 20 embeddedin beam 12, the curvature of beam 12 changes.

Data depicting the deflection of a cantilever beam, such as beam 12, isshown in FIG. 3. The data shown in FIG. 3 were taken using a non-contactinterferometer system (not shown) at ambient room temperature andpressure. Curvature of a cantilever beam ultimately depends on thetemperature profile along the beam. Temperature measurements taken alongbeam 12 show that the temperature profile along such beam is notconstant. The temperature profile changes, depending on many factors,including local heat generation, local curvature (which is notconstant), and ambient pressure (unforced air convection). Similarly,local heat generation along beam 12 depends on the local temperature andlocal grain structure in polysilicon heater 20. Despite the fact thatthe starting grain structure is fairly uniform across polysilicon heater20, this uniformity is eventually lost. Nonlinear resistance behavior ofpolysilicon features is well-known for un-suspended polysiliconstructures, but there are very few studies on suspended polysiliconstructures, so more studies are needed to understand all importantfactors in determining the profile of a thermally-actuated beam.However, it is well-known that, once heat is generated, the tip of acantilever, such as beam 12, can be controlled over large distances.

The fundamental effect that causes the change in the curvature of beam12 is known as a bi-morph effect. It is the result of differences inthermal expansion coefficients between two materials. As shown in FIG.4, a cantilever, such as beam 12, might contain many conducting(typically metal) and insulating layers (typically oxide). If a commonlyavailable IC process is used, the metal layers would be Aluminum, whilethe insulation layers would be silicon dioxide. As beam 12 is heated,the metal pieces expand much faster than the insulating layers, therebydecreasing the beam curvature.

Thus, the basis for the operation of microwave switch 10 is a bi-morpheffect. The height of the air-bridge 13 is chosen, such that for aparticular cantilever beam design (length, width, combinations ofthin-films), in an un-powered state (electrically ON-state), the tip ofbeam 12 would contact metal bridge 13, so that the signal-line (notshown) is connected to ground plane 11. For example, for a 200 μm longbeam, the data for which is shown in FIG. 3, the height of bridge 13 canbe chosen to be 25 μm or less. Although it is possible to havemetal-to-metal contact in this configuration, simply by increasing thecontact area at the tip of beam 12, because of sticktion issues, in anunpowered state, the tip of beam 12 is designed to havemetal-to-dielectric contact. (see the FIG. 5, the parts of 15 remainingon top of 9 will touch the bridge 13. In metal-to-metal contact therewon't be such dielectric pieces on top above 9.)

In addition, because of manufacturability issues, it is preferred tohave bridge heights of less than 15 μm. The basic consideration involvesthe determination of tolerable power dissipation at the powered state(electrically OFF-state, no connection between signal line and groundplane). The amount of actuation is determined by the power dissipation(equivalently generated heat) and the length of beam 12. Using the samepower, larger deflections can be obtained at the tip of longer beams,such as beam 12.

Another issue, which must be considered for the design of switch 10 isthe ON-state and OFF-state capacitance ratio of switch 10. It isdesirable to have high capacitance ratios, for example 100:1, to assurelower loss in the ON-state and high-isolation in the OFF-state. ON-statecapacitance can be increased by increasing the contact area, increasingthe dielectric constant of the material between metal layers in contactareas and decreasing the thickness of the dielectric layer. As discussedabove, if desired, it is possible to design the contact area (15 in FIG.4 shows the contact area) between beam 12 and bridge 13 to havemetal-to-metal contact. On the other hand, OFF-state capacitance dependson the separation of contact surfaces and the area of contact surface.It is preferable to have as much separation as possible in theOFF-state, but the amount of separation is limited by available power,length of beam and fabrication limits.

Switch 10 can also be used as a tunable capacitor. Switch 10 provides acapacitance with a huge capacitance ratio. However, it should be pointedout that the cantilever architecture is more suitable for the binaryoperation of a switch, rather than the more demanding continuousoperation of a tunable capacitor. A thermally actuated fixed-fixed beamis better for tunable capacitor applications.

FIGS. 2 b and 2 c show the preferred embodiment of a series tunablecapacitor 40 of the present invention, but without mechanical supportsbeing illustrated.

A polysilicon heater 41 is employed at the backside of the lower plate42, as shown in FIG. 2(c). The connections 43 to polysilicon heater 41are formed using a first metal layer (again 43 is a part of the firstmetal layer). The variable capacitance is obtained between the secondmetal layer (top surface 44 of lower plate 42) and the third metal layer(45 shows the third metal layer), which forms the upper plate 45. Upperplate 45 is fixed, but lower plate on beam 42 can be actuated by using abi-morph effect and polysilicon heater 41 buried within lower plate 42.

It should be noted that fixed-fixed beams can potentially buckle in bothdirection, i.e., into silicon or away from silicon. But, it has alsobeen found that if a field-oxide layer is used, a very large percentageof fixed-fixed beams buckle away from silicon. A field-oxide layer(shown as 17 in FIG. 5) is a relatively thick thermally grown silicondioxide layer which is under large compressive stress. If a field-oxidelayer is incorporated into the beam structure of capacitor 40, it wouldlie directly on the surface (not shown) of silicon wafer 25. Therefore,once the beam 42 is released, it would be the bottom layer, i.e., fieldoxide layer 17 underneath beam 42. If this layer is omitted, specialprecautions must be taken to assure the buckling direction of beam 42.In this case, the desired direction is away from surface of siliconwafer 25, or towards the upper plate 45.

Inclusion of a field-oxide layer has some undesired effects as well.Since it is so thick and significantly increases the stiffness of beam42, it also increases the power levels necessary to achieve desiredcapacitance ratio. When beam 42 buckles, it has a well-known raisedcosine profile, but since it is not an ideal fixed-fixed beam, the realbeam profile is fairly difficult to predict. This is especially true ifbeam 42 is much wider than polysilicon heater 41. The high frequencyconnection 46 to lower plate 42 can be changed from a straightconnection, as shown FIG. 2(b) to connections to the edges. This wouldincrease the reflection, but the thermo-electro-mechanical problem wouldbecome more manageable by simply assuming an ideal fixed-fixed beam.

The preferred capacitive embodiment of the present invention shown inFIGS. 2(b) and 2(c) uses a coplanar configuration. Ground planes 47 areformed using a second metal layer (not shown). Upper electrode 45 isfully supported by a mechanical support layer 48, and has a singleelectrical contact 49 to signal line of the output port (see FIG. 2(b).

The capacitance of capacitor 40 is varied by changing the powerdissipation in lower plate 42, whose maximum deflection decreases inresponse to increased heat from heater 41. The capacitance density alsochanges with the location of lower plate 42, since upper plate 45remains flat as lower plate 42 develops a raised-cosine shape. Thecapacitance per unit length (measured in vertical direction to heaterdirection) is calculated in closed form. Maximum to minimum capacitanceratios higher than 10:1 and a quality factor of more than 50 can beachieved with this architecture.

Although the switch and variable capacitor embodiments of the presentinvention shown in FIGS. 2(a) to 2(c) use thermal actuation, the presentinvention can also be implemented using electrostatic actuation. Withelectrostatic actuation, the third metal layer is kept fixed, while themoveable membrane is formed using layers available in a semiconductorprocess alone. A preferred embodiment of an electrostatically actuatedshunt switch 50 according to the invention is shown in FIG. 2(d). Theconstruction of the electrostatically actuated shunt switch 50 isgenerally the same as switch 10 shown in FIGS. 1 and 2(a), except asexplained below.

A moveable beam 50 consists of at least three metal pieces, 51, 52, 53,formed on the second metal layer encapsulated in a membrane formed byinter-layer dielectric films. Metal pieces 51 and 52 are used forelectrostatic actuation. They are connected to a voltage source (notshown) which is an integrated circuit located elsewhere on wafer 25.Metal piece 53 closes a gap 62 between two signal strips 60 and 61directly above metal piece 53, once beam 50 is pulled-up byelectrostatic actuation. Ideally, there is no dielectric on the surfaceof metal piece 53 so as to allow metal-to-metal contact between metalpiece 53 and signal strips 60 and 61. To minimize sticktion, it ispossible to add a thin layer of dielectric cover on metal piece 53. Allthree metal pieces, 51, 52 and 53 are typically encapsulated indielectric films (typically oxide), but to allow free vertical motion ofbeam 50, metal piece 53 is isolated from an overlaying dielectric filmmembrane 56 by cuts in such film shown by openings 54. Additionaletch-holes 55 in dielectric membrane 56 are added to facilitate theformation of a cavity 57.

A microwave waveguide is formed on third metal layer by using metalpieces, 58, 59, 60, and 61. Here again, such pieces form a coplanarwaveguide configuration including ground planes 58 and 59 and signalplanes 60 and 61. With gap 62 between signal planes 60 and 61, a signalcannot be transmitted. Ground planes 58 and 59 act as upper electrodesfor electrostatic actuation. So, when a transmission through signalplanes 60 and 61 is desired, beam 50 is pulled up by applying a voltagehigher than the threshold voltage of the switch. Ground planes 58 and 59are connected to circuit vias 63 and 64. These vias are formed as a partof third metal layer right above contact pads 64. Hence, circuit vias 63and 64 are electrically connected to integrated circuits elsewhere onthe wafer. Finally, ground planes 58 and 59 and signal planes 60 and 61are supported by the mechanical support layer 23.

FIGS. 6(a) through 6(i) illustrate a preferred fabrication process formaking the preferred embodiment of switch 10 of the present invention.This preferred process is based on semiconductor thin film depositionand photolithography processes, which are well known prior art. Otherfabrication sequences which are obvious to those skilled in the art arealso within the scope of the present invention.

The preferred embodiment of the electromechanical switch is fabricatedusing a semiconductor process in which a polysilicon layer, a firstmetal layer, and a second metal layer are deposited on a silicon wafer.By convention, in semiconductor processes, the layers are namedaccording to their order of deposition. The first metal layer is theclosest to the silicon substrate among metal layers, although it may bedeposited on top of multiple layers of polysilicon. All the conductivelayers are separated by insulating layers.

FIG. 6(a) shows a cross-sectional view of a completed semiconductor chip26. For thermal actuation at least one polysilicon layer 20 is needed,but other, resistive layers, which are typically used to form resistors,can be used as well. In CMOS processing, substrate 25 is silicon, butwith proper process changes at substrate at etch step, it is possible tofabricate similar devices on GaAs, SiC or other exotic substratematerials as well.

Another important consideration is the use of vias 27 (ie., cuts ininsulating layers) in a given process technology. To increase yield, theIC design rules set by a given foundry may be very restrictive. It isessential to have the capability of dielectric stacked vias, which candirectly expose substrate material for the fabrication sequence to beuseful. Although there are several foundries allowing such viaformations, typically, IC stacked vias are discouraged to improve theplanarity of layers. If such vias are not allowed in an IC process, anadditional masking layer is necessary to cut through the insulatinglayers 15, 17, 18 and 19 shown in FIG. 6(a).

In FIG. 6(b), a thick sacrificial layer 30 is patterned in area 14 (seeFIG. 5), that defines the cavity which allows free movement of beam 12.The thickness of sacrificial layer 30 is determined by designrequirements and fabrication limits. Photoresist, polymers and evenmetals can be used as sacrificial layer 30. It is preferable to usephotosensitive materials which can be removed easily layer, thereforephotoresists, especially thick varieties such as AZ 4600 series, AZ 9600series, and Shipley 220 series can be used to achieve 3-20 μm thickfeatures with fairly good aspect ratio. Since aspect ratio is notcritical for this application, resist and regular contact lithographywould also be acceptable for this step.

FIG. 6(c) shows the next step of forming the mold necessary forelectroplating. For this step, a seed layer 31 is deposited. Since goldis the preferred third metallization layer, seed layer 31 includes anadhesion and gold layer. A thin layer (100-300 A) of chromium ortitanium can be used for this purpose. If desired, a stack of Cr/gold/Crcan be used to minimize any step coverage issues. Preferably, goldthickness is 1000 A-3000 A. Both of these materials 31 can be depositedusing either evaporation or sputtering. Proper sputter clean-up shouldthen be performed to remove native oxide in exposed surfaces of secondlevel metal pads prior to seed layer deposition. This greatly improvescontact resistance and repeatability.

As shown in FIG. 6(d), once seed layer 31 is deposited, a second layerof thick resist is used to form a mold 32 for subsequent gold plating.Again, the same variety of resists can be used to form mold 32. Minimumfeatures should be larger than 5 μm at this step. Resist thicknessshould be more than the cavity height, to minimize lithography problems.Uniform resist thickness is hard to achieve by spin casting, but it isnot necessary anyway. For 5 μm thick gold deposition, it would bepreferable to have resist thickness of more than 5 μm. To lower cost,this sequence does not include any chemical-mechanical-polishing (CMP)step after gold deposition. It is also important not to overplatestructures.

In FIG. 6(e), about 5 μm thick gold is electroplated on wafer 25 throughthe exposed areas to form metal conductive bridge 13. This can be doneusing many available non-cyanide based gold plating solutions.

The step shown in FIG. 6(f) consists of three minor steps. First, resistmold 32 is stripped, and then seed layer 31 is partially removed, sinceseed layer 31 can not be removed under bridge 13. Preferably, both ofthese steps are done using dry etching systems. If cavity 14 is definedusing another resist layer, it is important to assure that it is wellcovered during the resist mold 32 strip operation. Oxygen plasma is canbe used to ash resist mold 32. Similarly, sputter etch can be used tostrip metal seed layer 31.

Finally, a superstrate 23 is deposited on top of switch 10, as shown inFIGS. 4 and 5. Several different materials can be used for this purpose.Polyimides, such as Epo-Tek 600 or DuPont's Pyralin, can bescreen-printed on this area. Several good alternatives are emerging fromhigh density interconnect (HDI) area, especially photoimageable versionsof sequentially build-up microvia organic substrates are very promising.Examples of such substrates include DuPont's dry film ViaLux 81,Vantico's liquid Probelec 81, Enthone's liquid Envision PDD 9015,MacDermid's liquid Macuvia-C, Shipley Royal's Aspire MultiPosit 2000 andDynaVia 2000. Most of these materials have glass transition temperaturesless than 200° C. For better coverage, liquid ones are preferable, butit has been observed that steps as high as 20 μm can be covered veryeasily by dry film varieties as well. Typically, the thickness of thesefilms can vary between 10 to 100 μm in a single coat. If the cavitycannot be stabilized mechanically in a single coat, as many coats asneeded must be applied over the cavity area. Typically, for a cavityheight of <20 μm, superstrate 23 height of 50 to 100 μm is enough.Finally, BCB (benzocyclobutene)-based polymers such as Dow Chemical'sCyclotene family can be used for this purpose as well. Compared tomicrovia dielectrics, BCB has lower loss at high frequencies (>1 GHz)and also lower dielectric constant (˜2.7), but typically the filmthickness is less than 10 μm per coat. Therefore, it would require moreprocessing.

In FIG. 6(g), the backside 36 of substrate 25 is patterned to form amask 35 by using front to back alignment to expose only the part ofsubstrate 25, which needs to be removed from back 36. The front side ofsubstrate 25 is also spray coated to minimize any interactions to withthe etchant, such as XeF2.

FIG. 6(h) shows selective removal of silicon substrate 25 from area 24using mask 35. For silicon substrates, numerous etching techniques canbe employed. The preferred approach is the use of pulsed XeF2 etchbecause of it is very high selectivity to silicon. XeF2 is an isotropicetchant. The etch surface gets rougher and less predictable as the etchgoes on, therefore thinner substrates are preferable at this step. Forsubstrates other than silicon, the etch technique must be changedaccordingly.

Finally, FIG. 6(i) is a cross-sectional view of electromechanical switch10 after removal of the sacrificial film 30 which defines air-cavity 14.Once the silicon of substrate 25 is completely removed in the designatedarea 24, beam 12 is released by removing the photoresist 30 that fillscavity 14. This can be done using a standard wet resist stripperapplication, followed by an oxygen plasma application to completelyclean cavity 14. As cantilever beam 12 is released, it curves or bucklesin cavity 14 so as to touch the third metal layer, bridge 13.

While the invention has been described in the context of a preferredembodiment, it will be apparent to those skilled in the art thatnumerous modifications may be made without departing from the true scopeof the invention, leading to numerous alternative embodiments.Accordingly, it is intended by the appended claims to cover allmodifications of the invention, which fall within the scope of theinvention.

1. A method of forming an electromechanical device comprising the stepsof: fabricating a first beam and an integrated circuit monolithically ona semiconductor substrate using standard semiconductor process flows;patterning above the first beam a sacrificial material; fabricating asecond beam by performing the steps of: depositing a first conductivematerial on the sacrificial material by means of sputtering orevaporation; patterning a sacrificial mold for a second conductivelayer; electrodepositing a thick second conductive layer over the mold;removing the sacrificial mold and an excess amount of the firstconductive layer underneath the mold; depositing and patterning amechanical support layer on top of the device; patterning and etchingthe semiconductor substrate from a backside of the substrate; andreleasing the first beam so as to be movable by removing the sacrificialmaterial on top of the first beam to form an air cavity above the firstbeam, the first beam being attached to the substrate at one or morepoints.
 2. The method of forming a device as recited in claim 1, whereinthe step of depositing the second conductive layer is followed by a stepof polishing the conductive layer.
 3. The method of forming a device asrecited in claim 1, wherein the steps of depositing the first and secondconductive layers are each repeated after a dielectric layer isdeposited and patterned over each of the first and second conductivelayers.
 4. The method of forming a device as recited in claim 2, whereinthe dielectric is a mechanical support layer.
 5. The method of forming adevice as recited in claim 1, wherein the substrate is selected from thegroup of semiconductor materials consisting of Si, SiGe and GaAs.
 6. Themethod of forming a device as recited in claim 3, wherein the mechanicalsupport layer is selected from the group consisting of screen-printedpolyimide, a photoimageable polymer, and a dry-etchable polymer.
 7. Themethod of forming a device as recited in claim 1, further comprising thestep of forming a polysilicon heater in the movable beam for heating andthereby actuating the moveable beam.
 8. The method of forming a deviceas recited in claim 1, wherein the first beam is fabricated from aplurality of conductive layers selected from the group consisting ofpolysilicon, aluminum and copper, and from a plurality of dielectriclayers selected from the group consisting of doped silicon dioxide,undoped silicon dioxide, a form of silicon nitride, and a low-kdielectric.
 9. The method of forming a device as recited in claim 1,wherein the second beam is fabricated from a plurality of conductivelayers selected from the group consisting of gold, copper, silver,platinum, titanium, tungsten, aluminum, nickel, and alloys thereof. 10.The method of forming a device as recited in claim 1, wherein, the stepof fabricating the first beam includes locating a conductive film at acontact area of the first beam, thereby allowing metal-to-metal contactbetween the first beam and the second beam.
 11. The method of forming adevice as recited in claim 1, wherein, the step of fabricating the firstbeam includes locating a dielectric film at a contact area of the firstbeam, thereby allowing metal-to-dielectric contact between the firstbeam and the second beam.
 12. A method of forming an electromechanicaldevice comprising the steps of: fabricating a moveable beam and anintegrated circuit on a semiconductor substrate using standardsemiconductor process flows; patterning an air-gap above the movablebeam using a sacrificial material; fabricating a fixed beam by:depositing a thick conductive film on the sacrificial material by meansof sputtering or evaporation; patterning the conductive film viastandard photolithography; and etching the conductive film; depositingand patterning a mechanical support layer on top of an area covered bythe device; patterning and etching the semiconductor substrate from abackside of the substrate; and releasing the moveable beam by removingthe sacrificial layer placed on top of the moveable beam to form an aircavity above the first beam, the movable beam being attached to thesubstrate at one or more points.
 13. The method of forming a device asrecited in claim 12 further comprising the step of forming a polysiliconheater in the movable beam for heating and thereby actuating themoveable beam.
 14. The method of forming a device as recited in claim 12wherein, the step of fabricating the movable beam includes locating aconductive film at a contact area of the moveable beam, thereby allowingmetal-to-metal contact between the movable beam and the fixed beam. 15.The method of forming a device as recited in claim 12 wherein, the stepof fabricating the movable beam includes locating a dielectric film at acontact area of the moveable beam, thereby allowing metal-to-dielectriccontact between the movable beam and the fixed beam.
 16. The method offorming a device as recited in claim 12, wherein the mechanical supportlayer is selected from the group consisting of a screen-printedpolyimide, a photoimageable polymer, and a dry-etchable polymers.